Adenosine receptors in brain membranes: Binding of N⁶-cyclohexyl[³H]adenosine and 1,3-diethyl-8-[³H]phenylxanthine

N⁶-Cyclohexyl[³H]adenosine ([³H]CHA) and 1,3-diethyl-8-[³H]phenylxanthine ([³H]DPX) bind to adenosine receptors in brain membranes. The agonist [³H]CHA has high affinity in both bovine and guinea pig brain (Kd, 0.7 nM and 6 nM, respectively). [³H]CHA binding kinetics are slow (dissociation t1/2; 60 min); binding is much higher at 25°C than at 0°C and is inhibited by guanine nucleotides. Potencies of nucleosides and xanthines in competing for [³H]CHA sites indicate that specific binding is entirely to A₁ adenosine receptors. In bovine brain, the antagonist [³H]DPX exhibits high-affinity binding (Kd, 5 nM) to the same A₁ receptors that bind [³H]CHA. Binding kinetics are rapid (dissociation t1/2, 1 min), and binding is moderately higher at 0°C than at 25°C. In guinea pig brain, [³H]DPX binding has only moderate affinity (Kd, 50 nM), and about 60% of specific binding is to sites that resemble A₂ adenosine receptors.